The present invention relates to chemical mechanical polishing (CMP), and more particularly, to pH control during a slurry free CMP process.
Chemical mechanical polishing (CMP) has emerged as a crucial semiconductor technology, particularly for devices with critical dimensions smaller than 0.3 microns. CMP machines typically include a means of holding a wafer or substrate to be polished (also referred to as a xe2x80x9cwafer chuckxe2x80x9d), a polishing pad, and a means to support the pad (also referred to as a xe2x80x9cplatenxe2x80x9d). In many conventional CMP machines, an abrasive slurry is required for polishing and is delivered either directly to the surface of the pad or through holes and grooves in the pad directly to the surface of the wafer. The polishing pad is not typically abrasive. The control system on the CMP machine causes motors to press the surface of the wafer against the pad surface with a prescribed amount of force.
One of the recent developments in CMP technology is slurry free fixed abrasive pad polishing. In this method, an abrasive slurry is not used. Instead, the abrasive material is fixed and integrated into the polishing pad. An example of such a system is described in U.S. Pat. No. 6,069,080 to James et al. The advantage of this type of system is that large amounts of abrasive slurry are not needed, thereby also eliminating the need for time consuming cleaning steps. Although an abrasive slurry is not used, an aqueous fluid is used to facilitate the polishing action. In the ""080 patent, the aqueous fluid is maintained at a constant pH.
Another concern in the CMP process is the accurate detection of the endpoint of the CMP process. In many important applications, the CMP process is used to remove an oxide layer until an underlying stop layer is reached, such as a nitride. For example, the use of a nitride as a stop layer underneath an oxide layer is commonly used in the formation of shallow trench isolation structures. However, it is still relatively difficult to accurately detect when the nitride stop layer has been reached.